Step 1Peel and rinse the turnips and rutabagas. Pat them dry and grate them into a large bowl using the largest holes of a box grater. You should have 5 1/4 cups of grated vegetables; if not, grate a little more.
Step 2Slice the onion as finely as possible, add it to the grated vegetables, and toss to combine. Set aside 5 to 10 minutes to let the flavors blend.
Step 3For the vinaigrette, whisk together the lemon juice, salt and pepper in a small bowl, then whisk in the olive oil. Set aside 1 tablespoon vinaigrette for the watercress and pour the remaining dressing over the grated vegetables and toss. Adjust the seasoning, adding more lemon juice or olive oil to taste if necessary.
Step 4Toss the watercress with 1 tablespoon of the vinaigrette and a couple grinds of white pepper. Arrange the vegetables on a platter, surround them with the dressed watercress and serve.